In a class of conventional mass spectrometry techniques known as "MS/MS" methods, ions (known as "parent ions") having mass-to-charge ratio within a selected range are isolated in an ion trap. The trapped parent ions are then allowed, or induced, to dissociate (for example, by colliding with background gas molecules within the trap) to produce ions known as "daughter ions." The daughter ions are then ejected from the trap and detected.
For example U.S. Pat. No. 4 736 101 issued Apr. 5, 1988, to Syka, et al., discloses an MS/MS method in which ions (having a mass-to-charge ratio within a predetermined range) are trapped within a three-dimensional quadrupole trapping field. The trapping field is then scanned to eject unwanted parent ions (ions other than parent ions having a desired mass-to-charge ratio) consecutively from the trap. The trapping field is then changed again to become capable of storing daughter ions of interest. The trapped parent ions are then induced to dissociate to produce daughter ions, and the daughter ions are ejected consecutively (sequentially by m/z) from the trap for detection.
In order to eject unwanted parent ions from the trap prior to parent ion dissociation, U.S. Pat. No. 4,736,101 teaches that the trapping field should be scanned by sweeping the amplitude of the fundamental voltage which defines the trapping field.
U.S. Pat. No. 4,736,101 also teaches that a supplemental AC field can be applied to the trap during the period in which the parent ions undergo dissociation, in order to promote the dissociation process (see column 5, lines 43-62), or to eject a particular ion from the trap so that the ejected ion will not be detected during subsequent ejection and detection of sample ions (see column 4, line 60, through column 5, line 6).
U.S. Pat. No. 4,736,101 also suggests (at column 5, lines 7-12) that a supplemental AC field could be applied to the trap during an initial ionization period, to eject a particular ion (especially an ion that would otherwise be present in large quantities) that would otherwise interfere with the study of other (less common) ions of interest.
It is conventional to perform "higher order MS/MS" operations (sometimes referred to as "(MS).sup.n " operations) in which products of daughter ions (i.e., additional generations of daughter ions) such as "granddaughter ions" are trapped and then excited for detection. For example, in an (MS).sup.3 method (i.e., an MS/MS/MS method), a selected parent ion is dissociated and its daughter ions are trapped and then induced (or permitted) to dissociate (or otherwise react) to produce a species of trapped granddaughter ions. The trapped granddaughter ions are then ejected from the trap for detection.
For another example, in an (MS).sup.4 method (i.e., an MS/MS/MS/MS method), a selected parent ion is dissociated and its daughter ions are trapped and then induced (or allowed) to dissociate (or otherwise react) to produce a species of trapped granddaughter ions, and the granddaughter ions are then induced (or allowed) to dissociate (or otherwise react) to produce a species of trapped great-granddaughter ions. The trapped great-granddaughter ions are then consecutively ejected from the trap for detection.
U.S. Pat. No. 4,686,367, issued Aug. 11, 1987, to Louris, et al., discloses another conventional mass spectrometry technique, known as a chemical ionization or "CI" method, in which stored reagent ions are allowed to react with analyte molecules in a quadrupole ion trap. The trapping field is then scanned to eject product ions which result from the reaction, and the ejected product ions are detected.
European Patent Application 362,432 (published Apr. 11, 1990) discloses (for example, at column 3, line 56 through column 4, line 3) that a broad frequency band signal ("broadband signal") can be applied to the end electrodes of a quadrupole ion trap to simultaneously resonate all unwanted ions out of the trap (through the end electrodes) during a sample ion storage step. EPA 362,432 teaches that the broadband signal can be applied to eliminate unwanted primary ions as a preliminary step to a CI operation, and that the amplitude of the broadband signal should be in the range from about 0.1 volts to 100 volts.
However, conventional (MS).sup.n and CI methods are capable only of obtaining information of limited scope regarding each sample of interest. It would be desirable to obtain a broader range of information regarding a sample than can be obtained from such conventional methods. To minimize the time required to analyze a sample, and to maximize sample information, it would also be desirable to obtain such information in a manner in which daughter ions of interest, or products of daughter ions of interest, or both, are selectively resonated for detection. However, until the present invention, it was not known how simultaneously to achieve all these objectives in an ion trap.